Arc chute with magnetic blowout means having larger phase lag for vertex of chute than the throat

ABSTRACT

Discloses an arc chute for an alternating current circuit breaker which comprises a plurality of electromagnets for developing magnetic fields in different parts of the chute. The magnetic field developed in and beyond the vertex region of the chute has a much higher phase lag with respect to arc current than the magnetic field developed in the throat region of the arc chute. This latter magnetic field is nearly in phase with the arc current.

United States Patent Inventors Appl. No.

Filed Patented Assignee Gerhard Frind Glenolden. Pa.; Rudolf Hunziker, Collingswood, NJ.; Richard M. Korte, Media, Pa. 827,720

May 26, 1969 July 6; 1971 v General Electric Company ARC CHUTE WlTH MAGNETIC BLOWOUT MEANS HAVING LARGER PHASE LAG FOR VERTEX OF CHUTE THAN THE THROAT 5 Claims, 6 Drawing Figs.

US. Cl 200/147 R .lnt.Cl ....H0lh 33/18 Field of Search 200/ l 47 [56] References Cited UNITED STATES PATENTS 2,828,380 3/1958 Lingal 200/l47 X 2922,926 1/1960 Fehlingmm. 200/147 X 3,050,602 8/1962 Korte et al. 200/147 Primary Examiner-Robert K. Schaefer Assistam Examiner-Robert A. Vanderhye Attorneys-.l. Wesley Haubner, William Freedman, Frank L.

Neuhauser, Oscar B. Waddell and Melvin M. Goldenberg ABSTRACT: Discloses an arc chute for an alternating current circuit breaker which comprises a plurality of electromagnets for developing magnetic fields in different parts of the chute. The magnetic field developed in and beyond the vertex region of the chute has a much higher phase lag with respect to are current than the magnetic field developed in the throat region of the arc chute. This latter magnetic field is nearly in phase with the arc current.

ARC CI-IUTE WITI-I MAGNETIC BLOWOUT MEANS HAVING LARGER PHASE LAG FOR VERTEX F CIIUTE THAN THE THROAT This invention relates to an alternating current electric circuit breaker of the type which relies upon magnetic blowout means for driving a circuit interrupting are into an arc extinguishing device, referred to hereinafter as an arc chute.

More specifically, the invention is concerned with the type of arc chute in which a plurality of magnetic blowout coils are located along the conductive arc runners of the chute for developing magnetic fields for accelerating movement of the are along the runners into the interior of the chute. Examples of this general type of arc chute are shown in U.S. Pat. Nos. 2,901 ,579Simpson and 3,050602Korte et al., both assigned to the assignee of the present invention. These blowout coils are connected in series with the are as it moves into the chute, and the magnetic field resulting from energization of each blowout coil by the alternating arcing current is an alternating magnetic field.

The usual arc chute of this type comprises insulating sidewalls extending between the arc runners and plates of insulating material extending transversely of the sidewalls with edges for engaging the arc as it moves into the chute. These edges define a vertex region which the arc enters and passes beyond as it moves into the chute. Motion of the are beyond, or behind, the vertex region forces the arc to assume a zigzag path around the edges of the insulating plates.

It has heretofore been recognized that the alternating magnetic field in this vertex region should desirably have some phase lag with respect to the alternating arcing current so as to provide an appreciable magnetic force on an arc in this region during the period immediately preceding current zero. But it has not been appreciated that the phase lag which is desirable at the vertex region is quite undesirable in the throat region of the arc chute, i.e., where the runners have their minimum spacing. We find that in this throat region, a magnetic field with a large phase lag can interfere with prompt arc transfer from the contacts and can force an are on the runners back toward the contacts.

Accordingly, an object of our invention is to provide for such an arc chute a magnetic blowout system which can provide in and beyond the vertex region a magnetic field having a large phase lag with respect to arcing current but which provides in the throat region a magnetic field only slightly out of phase with the arcing current.

In carrying out our invention in one form, we provide an arc chute with first and second spaced-apart arc runners. Following initiation of the are, a first terminal of the arc is transferred to the first runner, and thereafter the second terminal of the arc is transferred to the second runner. For propelling the arc, we provide a plurality of blowout electromagnets which are connected in series with the are as it moves into the chute. One of these electromagnets is used for developing a magnetic field in the throat region of the arc chute, and others are used for developing a magnetic field in and beyond the vertex region of the chute. The electromagnet that develops the magnetic field in the throat region has pole pieces which substantially cover that predetermined portion of the throat region occupied by the arc at the time its second terminal normally transfers to the second arc runner. These pole pieces are of a highly laminated construction that maintains the magnetic field in said predetermined portion of the throat region nearly in phase with the arcing current. The pole pieces of said other electromagnets are of a solid or only slightly laminated construction that causes the magnetic field developed by these magnets in and beyond the vertex region to be greatly out of phase with the arcing current.

For a better understanding of the invention, reference may be had to the following description taken in conjunction with the accompanying drawings, wherein:

FIG. I is a side elevational view partly in section showing an electric circuit breaker embodying one form of the invention.

FIG. 2 is a cross-sectional view taken along the line 2-2 of FIG. 1.

FIG. 3 is an end view of the arc chute of FIG. 1.

FIG. 4 is a plan view of one of the blowout electromagnets used in the interrupter of FIG. 1.

FIG. 5 is a view taken along the line 5-5 of FIG. 4.

FIG. 6 is a graphical representation of certain electrical and magnetic relationships present in the arc chute of FIG. 1.

Referring now to FIG. 1 of the drawing, the circuit breaker shown therein comprises a pair of terminal bushings I and 2, both of which are fixed in position relative to the supporting frame of the circuit breaker. The bushing 2 comprises a downwardly extending conductive stud 3 at the lower end of which a movable conductive switch blade 4 is mounted by means of a fixed pivot 5. At its outer end, the blade 4 carries suitable circuit controlling contacts such as a main current carrying contact 6 and an arcing contact 7.

Bushing 1 comprises a conductive stud la to which a downwardly extending conductive member 8 is electrically connected. Attached to this conductive member 8 is a curved contact retaining member 9 which coacts with the member 8 to form a holding pocket for receiving the anchored ends of main stationary current carrying contact fingers 10. These fingers are pivotally mounted on a curved portion 12 of the conducting member 8 and are biased for limited rotative wiping movement in a closing direction by means of suitable compression springs 9a. These compression springs provide for high pressure circuit closing engagement between the stationary current carrying contact 10 and the movable main current carrying contact 6.

The movable arcing contact 7 cooperates with stationary arcing contact 13, which is mechanically and electrically connected to the conducting member 8 by suitable clamping means 14. The material of which the arcing contacts 7 and 13 are made is capable of withstanding arcing and is also of relatively high resistivity in comparison to the material of the current carrying contacts 10 and 6. Accordingly, when the switchblade 4 is in its closed position as shown in FIG. 1 most of the circuit current flows through the current carrying contacts. It is only when the switchblade 4 is driven counter clockwise to open the breaker that the arcing contacts carry appreciable current. During contact opening action, the current carrying contacts part first, thereby diverting current through the arcing contacts which are still in engagement owing to their extensive wipe. Thereafter, the arcing contacts part and draw a circuit interrupting arc therebetween which is driven into an arc chute 15 and there lengthened, cooled and extinguished.

For driving the switchblade 4 counterclockwise to effect circuit interruption, a reciprocable operating rod 16 pivotally joined to the switchblade at point-17 is provided. When this operating rod is driven upwardly, it acts to move the switchblade counterclockwise to effect a circuit interrupting operation. The circuit can be reestablished by driving the operating rod downwardly to return the switchblade 4 in a clockwise direction to the closed position illustrated in FIG. 1. The operating rod 16, which is made of insulating material, is actuated by means of a suitable conventional operating mechanism (not shown).

The are chute 15 comprises a pair of sidewalls l8 and 19 constructed of appropriate are resistant and tracking resistant insulating material. As shown in FIG. 3, these sidewalls are clamped together in spaced apart relationship by means of insulating clamping strips 20 and 21. Each sidewall is provided with plurality of fins 22 projecting toward the other sidewall and arranged to interleave with the corresponding projecting fins on the other sidewall, thereby forming a sinous or zigzag passage as viewed either from the entrance end or from the exhaust end of the chute. The view from the latter end is illustrated in FIG. 3.

Referring to FIG. 2, the forward edges of the interleaving fins 22 taper toward the arc-initiation region at the right-hand end of the chute. The tapering edges of each adjoining pair of fins 22 intersect as viewed in FlG. 2 to form an entrance angle such asa at the entrance to the zigzag passage between the fins. The vertexes of these entrance angles all lie on a curvilinear line 23 (FIG. 1) which constitutes their locus. The region of the arc chute immediately adjacent this line 23 we refer to hereinafter as the vertex region of the arc chute.

For facilitating movement of the are into the arc chute, a pair of conductivearc runners 24 and 25are provided along opposite edges of the chute. The sidewalls 18 and 19 extend between these runners24 and 25.'As shown in FIG. 1, runners 24 and 25 extend generally transverse .to the axis of the arc and in divergent relationship with respect to each other from the arc-initiation region adjacent the separable contacts. The region of the arc chute where the diverging arc runners 24 and 25 are spaced apart by the least distance we refer to as the throat region of the arc chute. This throat region is designated 50in FIG. 1.

Projecting from and electrically connected to the upper runner 24 are two probes 51 of a refractory conductive material. The probes are located immediately adjacent but slightly spaced from the movable Switchblade 4. The purpose of these probes is to accelerate transfer of the upper arc terminal to the upper arc runner 24.. v

The are chute further comprises a first group of blowout electromagnets 26, 27 and 28 mounted adjacent the upperarc runner 24 and a second group of blowout electromagnets 29, 30 and 31 mounted adjacent the lower arc runner 25. Each of these blowout magnets comprises a generally U-shaped structure that comprises a pair of pole pieces 32 and 33 constituting the legs of the U and an interconnecting core member 34 between the legs of the U. Each blowout magnet straddles the chute with its pole pieces mounted at the outer sides of sidewalls 24 and 25 and its core member extending between the sidewalls on the right-hand side of the arc runners, as seen in H6. 1. Mounted around the cores of the blowout magnets are blowout coils 26a, 27a, 28], 29a, 30a and 31a. As shown, each blowout coil is round and its inside diameter is such that it snugly surrounds an insulating sleeve on the round cross sec tion core on which it is mounted. The blowout coils in the upper group are connected in series relationship with each other, and those of the lower group are also connected in series with each other. I

With respect to the electrical connection 'of the upper blowout coils, note that the upper arc runner 24 is divided into 'a'plurality of segments separated by insulating members 52 and 53. Blowout 'coil' 27a electrically bridges insulating member 52, andblowout coil 28a electrically bridges'insulating member 53. The first of the upper blowout coils 26a is electrically connected between the stud la and the first seg ment of the arc runner. la a similar manner, the lower blowout coils 30a and 31a electrically bridge insulating members 54 and 55 between segments of the lower runner. The first of the lower blowout coils 29a is electrically connected between the first segment of the lower runner and the conductive stud 3 by means of an electric conductor 57.

When the contacts of the circuit breaker are closed, the blowout coils are not in the power circuit and are consequently deener gized. When the contacts are separated to form an arc, movement of the are along the arc runners 24 and 25 into the chute connects these blowout coils in series with the are, as will soon appear more clearly. An important purpose of the blowout magnets is to propel the arc and accelerate'its movement along the runners into the interior of the arc chute. When a particular blowout coil is energized, the magnetic field produced between its pole pieces extends transversely of the arc column. This magnetic field reacts with the magnetic field surrounding the arc in a known manner to produce a resultant force that drives the are at high speed toward the interior of the chute. t

The pole pieces 32 and 33 of the live blowout magnets 27, 28, 29, 30 and 31 are'of a solid or nearly solid construction and contain only a few laminations at most. In a commercial form of the invention used for supplying the data in the next paragraph, each of these pole pieces contains two laminations, each being of the shape depicted in FIG. I and disposed in planes parallel to the plane of FIG. 1.

Referring to FIG. 6, when the blowoutcoil of any one of these electromagnets 27, 28, 30 or 31 is energized by an alternating current l, there is developed between its pole pieces an alternating magnetic field having a flux density varying in accordance with the curve B,, as measured in a region located a short distance from the outer end of the pole pieces. In FIG. 6, the-flux density B, and the current l are plotted against the same time scale. It will be seen that the flux density has a waveform approximating but not quite that of the sinusoidally varying current. Since the phase relationshipbetween current I and flux density B varies slightly during the course of each alternation in view of this difference in waveform, we will refer to the phase relationship between these quantities in terms of the number of electrical degrees which peak flux lags peak current. With these generally solid pole pieces, this phase lag between the peaks is a relatively large amount, about 60. The generally solid construction of these pole pieces allows relatively high eddy currents to be induced in the pole pieces, and these high eddy currents produce a relatively large amount of phase shift. A slightly greater phase shift can be obtained by using for magnets 27, 28, 30 and 31 solid pole pieces entirely free of laminations, and our invention comprehends such a construction.

'The pole piecesof the one remaining blowout magnet 26 are of a spirally wound-construction similar to that depicted in the aforesaid Korte et al. patent. In the form of the invention illustrated in FIGS. 4 and 5, each of these pole pieces is formed from a single ribbon of silicon steel that is coated with a suitable insulating varnish and is wound to form a tight coil. A suitable band of insulating material (not shown) surrounds each of the coils to prevent its unwinding. A hole 35 is drilled into each of the, coils at the location where the axis of core member 34 is to be located.

Core member 34 is a steel tube having a length equal to the predetermined distance between the two pole pieces. The core member 34 is surrounded by an insulating tube 37 which serves to insulate the core from the surrounding blowout coil 26a. Blowout coil 26a is assembled about the tube 37 on the core, after which this assembly is clamped between the two pole pieces by a through bolt 38 extending through the central opening in the core and the openings 35 in the pole pieces. The pole pieces are suitably coated with insulation 39 in all regions except where the core mernber engages the pole pieces.

When coil 26a of magnet 26 is energized by an alternating current, a magnetic field having a flux density varying'in accordance with curve B (P10. 6) is developed. This field extends between pole pieces 32 and 33 of magnet 26 and alternates at essentially the same frequency as the current. But unlike the field developed by each of the other blowout magnets, the alternating magnetic field developed by blowout magnet 26 is nearly in phase with the current through its blowout coil. Measurements made with this magnetic structure show that over most of the pole piece surface the magnetic field derived therefrom in the throat region of the arc chute lags the current by onlyabout 20 or less, as measured between current zero and fiux zero. The highly laminated character of the pole pieces of magnet 26 drastically limits the eddy current induced therein, thus limiting the flux lag to a very low value and also increasing the flux density in the space between the pole pieces.

The following is a more specific description of an interrupting operation. When switch blade 4 of FIG. 1 is driven counterclockwise to open the breaker, an arc is established at the arcing contacts 7 and 13 as soon as the contacts part. After the arc is established at the arcing contacts, its upper terminal, aided by the probes 51, quickly transfers to the upper arc runner thereby connecting the first of the upper blowout coils 26a in series with the arc. In the meantime, the movable switch blade 4 swings rapidly away from the stationary contact; and when it moves through an intermediate position depicted by dotted lines 58 in FIG. 1, the lower arc terminal transfers to the lower arc runner. The position of the are just prior to such transfer is depicted at 59. The magnetic field produced by the then-energized upper blowout coil 260 plays an important role in forcing a high-speed transfer of the lower arc terminal to the lower arc runner 25, as will soon be explained.

When the lower arc terminal transfers to the lower arc runner, the lower arc terminal moves toward the interior of the chute along the lower runner. When the arc is in the throat region 50, the primary magnetic field for moving the are into the chute is derived from the upper blowout coil 26a. As the are moves along the runners toward the interior of the chute, it successively inserts the blowout coils of the upper and lower groups into the power circuit in series with the arc, thus creating additional magnetic field for propelling the are into the chute.

As the arc tenninals move along the runners into the chute, the column of the arc moves freely into the chute until it encounters the intersecting forward edges of the interleaving fins 22 at the vertex region 23. Thereafter, further penetration of the arc into the arc chute causes the arc column to assume a zigzag form as it bends around the overlapping edges of the fins 22. This elongation of the arc and the cooling that results from the intimate engagement of the arc and fins are important factors contributing to successful interruption.

We find that it is important to successful interruption that a relatively high magnetic force be present on any are that is beyond the vertex region during the period immediately preceding current zero. This force is needed to force the zigzag are further into the chute and into intimate contact with cooler portions of the chute during the crucial period just prior to current zero. We are able to provide this high magnetic force during this period by using pole pieces that cause the flux that is present in and beyond the vertex region to lag the current by a large amount. in this respect, the pole pieces that provide the flux in and beyond the vertex region 23 are the generally solid pole pieces of blowout magnets 27, 28, 30 and 31 and these solid pieces, as previously pointed out, produce a flux lag of about 60 electrical degrees in and beyond the vertex region.

If the arc should reignite at current zero in a region covered by these solid pole pieces, the flux from the solid pole pieces acts for 60 electrical degrees in a direction to drive the are back toward the contacts. But we have found that despite this backward force, the arc column once it reaches the vertex region will not move back toward the contacts. The reason for this, we believe, is that when the arc has reached the vertex or is beyond the vertex, the current path through the arc chute is in the shape of a large loop bowing into the chute. The magnetic effect of current through a path of this shape is to lengthen the loop by driving the are further into the chute. This latter magnetic effect, referred to hereinafter as a magnetic loop effect, counteracts the backward-acting magnetic effect from the lagging magnetic field derived from the solid pole pieces, thus preventing the are from moving back toward the contacts under the influence of this backward-acting magnetic effect. The magnetic loop effect is able to prevail even though a relatively large phase lag in the magnetic field from the pole pieces is present adjacent the runners 24, 25.

The key region in which the magnetic field should have a large phase lag with respect to arcing current is the region beyond the vertex where the highest current arcs will be located during the peak current period just preceding final interruption. in the particular region beyond the vertex where an arc of maximum rated current will burn at the time of peak current just preceding final interruption, I provide a phase lag of at least 50 The flux density in this region varies with respect to current in essentially the manner depicted in the curve B ofFlG. 6.

Although a large phase lag in the magnetic field present in and beyond the vertex 23 has been found beneficial, as pointed out hereinabove, we find that in the throat region 50 a large phase lag is detrimental to successful interruption. As mentioned hereinabove, the magnetic field in this region is relied upon to transfer the arc from the moving contact onto the lower arc runner and to move the are out of the throat region. We find that to effectively perform these functions, the magnetic field in the throat region, especially in the portion of the throat region occupied by the arc just prior to the instant at which the lower arc terminal transfers to the lower runner, should be nearly in phase with the arc current, lagging the arcing current by no more than 30 degrees, In this regard, an inphase relationship in the time zone around peak current maximizes the force available for arc transfer and in the time zone around current zero minimizes the backward-acting magnetic force on the are. If the magnetic field is in the wrong direction with respect to the arc current when the arc is in this throat region, it will retard arc-transfer or will tend to force the are back on to the contacts if already on the runners. When the arc is in the throat region, there is no large loop force to counteract any backward-acting magnetic force. We find also, from high-speed photographs, that an arc in the throat region burns in the shape of a broad band, rather than in a distinct column, and such arcs do not develop high loop forces.

We are able to provide a magnetic field in the upper half of the throat region 50 that is nearly in phase with the arc current because we rely upon the blowout magnet 26 with the highly laminated spirally wound pole pieces for developing the magnetic field in this particular region. As pointed out hereinabove, over most of the pole piece surface this magnetic field from the blowout magnet 26 has a phase lag with respect to current of only about 20 electrical degrees or less, as measured between current zero and flux zero. With the magnetic field and the arcing current this closely in phase, we can consistently effect a prompt transfer of the lower arc terminal to the lower arc runner 25 and can avoid backward motion of an arc already on the runners.

Another advantage of the large phase lag that is present in and beyond the vertex region is that lower arc voltages are produced at peak currents. Arc voltage appears to be directly dependent on the product of arc current and the flux density of the transverse magnetic field. By forcing a delay in the flux density peak until well after peak current, we avoid a condition where peak current and peak flux density will be reached simultaneously, thus reducing the maximum arc voltage developed. This reduction in maximum arc voltage is accompanied by a corresponding reduction in the arc erosion of the arc chute that occurs at peak current. This desirably prolongs the life of the arc chute and also facilitates high current interruption by reducing the volume of the arcing products which are generated and must be effectively cooled and de-ionized.

While we have shown and described a particular embodiment of our invention, it will be obvious to those skilled in the art that various changes and modifications may be made without departing from our invention in its broader aspects; and we, therefore, intend herein to cover all such changes and modifications as fall within the true spirit and scope of our invention.

What we claim as new and desire to secure by Letters Patent of the United States is:

1. An arc chute for an alternating current electric circuit breaker of the magnetic blowout type comprising:

a. means defining an arc-initiation region in which an arc is initiated during an interrupting operation,

b. first and second spaced-apart metallic arc runners extending divergently with respect to each other from said arc-initiation region into said chute and providing paths along which the terminals of an arc move as the arc is driven into said chute,

. said chute having a throat region adjacent said arc-initiation region where said runners are spaced apart by a minimum distance,

. means for causing a first terminal of an arc initiated in said arc-initiation region to transfer onto said first arc runner and a second terminal later to transfer onto said second arc runner in said throat region,

e. a pair of spaced-apart sidewalls of insulating material extending generally parallel to the path followed by said are as it moves into said chute,

f. spaced-apart plates of insulating material extending transversely of said sidewalls and having edges extending at an acute angle to said sidewalls for engaging said are as it moves into said chute,

g. said edges defining a vertex region beyond which further motion of the are into the chute causes the arc to develop a zigzag configuration looping around the edges of said plates,

h. means for developing in substantially the entire portion of said throat region normally occupied by said are just prior to transfer of its second terminal to said second arc runner a magnetic field that extends transversely of said are, alternates at substantially the same frequency as the arcing current, and is out of phase no more than 30 electrical degrees with said arcing current, as measured between current zero and flux zero, (h' said means of (h) comprising first blowout magnetic means comprising pole pieces substantially covering said entire portion of said throat region normally occupied by said are just prior to transfer of its second terminal to said second arc runner,

i. and means for developing in the region of said chute beyond said vertex region a lagging magnetic field that extends transversely of an arc in said region beyond the vertex region, alternates at substantially the same frequency as the arcing current, and lags said arcing current by at least 50 electrical degrees, as measured between peak current and peak flux density,

j. said lagging magnetic field being present along a major portion of the length of an are located beyond said vertex region.

2. The are chute of claim 1 in which said pole pieces are highly laminated to reduce eddy currents.

3. The are chute of claim 2 in which said means for developing the lagging magnetic field in the region beyond said vertex region comprises second blowout magnet means having pole pieces with portions aligned with said region behind the vertex, each of said latter pole pieces having far fewer laminations capable of reducing eddy currents than the pole pieces of said first magnetic blowout means.

4. The are chute of claim 3 in which each of said latter pole pieces has no more than a few laminations capable of reducing eddy current.

5. The are chute for an alternating current electric circuit breaker of the magnetic blowout type comprising:

a. means defining an arc-initiation region in which an arc is initiated during an interrupting operation,

b. first and second spaced-apart metallic arc runners extending divergently with respect to each other from said arc-initiation region into said chute and providing paths along which the terminals of an arc move as the arc is driven into said chute,

c. said chute having a throat region adjacent said arc-initiation region where said runners are spaced apart by a minimum distance,

d. means for causing a first terminal of an are initiated in said arc-initiation region to transfer onto said first arc runner and a second terminal later to transfer onto said second arc runner in said throat region,

e. a'pair of spaced-apart sidewalls of insulating material extending generally parallel to the path followed by said are as it moves into said chute,

f. spaced-apart plates of insulating material extending transversely of said sidewalls and having edges extending at an acute angle to said sidewalls for engaging said are as it moves into said chute,

g. said edges defining a vertex region beyond which further motion of the are into the chute causes the arc to develop a zigzag configuration looping around the edges of said plates,

h. means for developing in substantially the entire portion of said throat region normall occupied by said are just rior to transfer 0 its secon terminal to said secon arc runner a magnetic field that extends transversely of said arc, alternates at substantially the same frequency as the arcing current, and lags said arcing current by only a small amount,

i. said means of (h) comprising first blowout magnet means comprising pole pieces highly laminated to reduce eddy currents substantially covering said entire portion of said throat region normally occupied by said are just prior to transfer of its second terminal to said second arc runner,

j. and means for developing in the region of said chute beyond said vertex region a lagging magnetic field that extends transversely of an are over a major portion of the arc length in the said region beyond the vertex, alternates at substantially the same frequency as the arcing current, and lags said arcing current by a relatively large amount compared to the flux lag in the throat region,

k. said means of (j) comprising second blowout magnet means comprising pole pieces with portions aligned with said region beyond the vertex region, each of said latter pole pieces having far fewer laminations capable of reducing eddy currents than the pole pieces of said first magnetic blowout means. 

1. An arc chute for an alternating current electric circuit breaker of the magnetic blowout type comprising: a. means defining an arc-initiation region in which an arc is initiated during an interrupting operation, b. first and second spaced-apart metallic arc runners extending divergently with respect to each other from said arc-initiation region into said chute and providing paths along which the terminals of an arc move as the arc is driven into said chute, c. said chute having a throat region adjacent said arcinitiation region where said runners are spaced apart by a minimum distance, d. means for causing a first terminal of an arc initiated in said arc-initiation region to transfer onto said first arc runner and a second terminal later to transfer onto said second arc runner in said throat region, e. a pair of spaced-apart sidewalls of insulating material extending generally parallel to the path followed by said arc as it moves into said chute, f. spaced-apart plates of insulating material extending transversely of said sidewalls and having edges extending at an acute angle to said sidewalls for engaging said arc as it moves into said chute, g. said edges defining a vertex region beyond which further motion of the arc into the chute causes the arc to develop a zigzag configuration looping around the edges of said plates, h. means for developing in substantially the entire portion of said throat region normally occupied by said arc just prior to transfer of its second terminal to said second arc runner a magnetic field that extends transversely of said arc, alternates at substantially the same frequency as the arcing current, and is out of phase no more than 30 electrical degrees with said arcing current, as measured between current zero and flux zero, (h'' said means of (h) comprising first blowout magnetic means comprising pole pieces substantially covering said entire portion of said throat region normally occupIed by said arc just prior to transfer of its second terminal to said second arc runner, i. and means for developing in the region of said chute beyond said vertex region a lagging magnetic field that extends transversely of an arc in said region beyond the vertex region, alternates at substantially the same frequency as the arcing current, and lags said arcing current by at least 50 electrical degrees, as measured between peak current and peak flux density, j. said lagging magnetic field being present along a major portion of the length of an arc located beyond said vertex region.
 2. The arc chute of claim 1 in which said pole pieces are highly laminated to reduce eddy currents.
 3. The arc chute of claim 2 in which said means for developing the lagging magnetic field in the region beyond said vertex region comprises second blowout magnet means having pole pieces with portions aligned with said region behind the vertex, each of said latter pole pieces having far fewer laminations capable of reducing eddy currents than the pole pieces of said first magnetic blowout means.
 4. The arc chute of claim 3 in which each of said latter pole pieces has no more than a few laminations capable of reducing eddy current.
 5. The arc chute for an alternating current electric circuit breaker of the magnetic blowout type comprising: a. means defining an arc-initiation region in which an arc is initiated during an interrupting operation, b. first and second spaced-apart metallic arc runners extending divergently with respect to each other from said arc-initiation region into said chute and providing paths along which the terminals of an arc move as the arc is driven into said chute, c. said chute having a throat region adjacent said arc-initiation region where said runners are spaced apart by a minimum distance, d. means for causing a first terminal of an arc initiated in said arc-initiation region to transfer onto said first arc runner and a second terminal later to transfer onto said second arc runner in said throat region, e. a pair of spaced-apart sidewalls of insulating material extending generally parallel to the path followed by said arc as it moves into said chute, f. spaced-apart plates of insulating material extending transversely of said sidewalls and having edges extending at an acute angle to said sidewalls for engaging said arc as it moves into said chute, g. said edges defining a vertex region beyond which further motion of the arc into the chute causes the arc to develop a zigzag configuration looping around the edges of said plates, h. means for developing in substantially the entire portion of said throat region normally occupied by said arc just prior to transfer of its second terminal to said second arc runner a magnetic field that extends transversely of said arc, alternates at substantially the same frequency as the arcing current, and lags said arcing current by only a small amount, i. said means of (h) comprising first blowout magnet means comprising pole pieces highly laminated to reduce eddy currents substantially covering said entire portion of said throat region normally occupied by said arc just prior to transfer of its second terminal to said second arc runner, j. and means for developing in the region of said chute beyond said vertex region a lagging magnetic field that extends transversely of an arc over a major portion of the arc length in the said region beyond the vertex, alternates at substantially the same frequency as the arcing current, and lags said arcing current by a relatively large amount compared to the flux lag in the throat region, k. said means of (j) comprising second blowout magnet means comprising pole pieces with portions aligned with said region beyond the vertex region, each of said latter pole pieces having far fewer laminations capable of reducing eddy currents than the pole pieces of said first magnetic blowout means. 